The present invention relates to an apparatus for driving a semiconductor laser which is used as a light source in a laser-beam printer, more particularly to a laser driving apparatus for ensuring stable periodic scanning operations.
In an example of a laser beam printer, when a laser beam emitted from a semiconductor laser scans a light-sensitive drum to record information thereon, it also scans a light-sensitive detector provided at one edge of the drum to obtain a scanning start pulse signal which defines a reference time point for the start of each scanning of the drum. Such a laser-beam printer is disclosed in, for example, a Japanese Patent Application laid-open No. 2826/1975 laid open on Jan. 13, 1975.
As is well known, the light-sensitivity of a light-sensitive drum used in a laser-beam printer degrades with time, that is, the light-sensitivity of the drum decreases as the drum is used longer. In order to maintain the print quality constant, it is required to increase the output or the intensity of the laser beam in accordance with the decreasing light-sensitivity of the drum. When the light-sensitivity of the drum becomes lower than a certain level, it is impossible to obtain a desired print quality, and therefore the drum is replaced by a new drum. With the new drum the output or the intensity of the laser beam has to be lowered to obtain the same print quality, since the new drum is considerably high in the light-sensitivity as compared with the deteriorated one. As mentioned above, the intensity of the laser beam incident upon the detector is changed in accordance with the light-sensitivity of a drum, and therefore the output of the detector is also changed. Such change in the output of the detector fluctuates the time at which the scanning start pulse signal is generated, thereby fluctuating the starting time point for each scanning operation for the drum.
Further, when the drum has a relatively high light-sensitivity and the intensity of the laser beam is too lowered, the output of the detector becomes so small that it is required to increase the gain of an amplifier for amplifying the output of the detector. Such an increase in the gain of the amplifier inevitably degrades the frequency-characteristic of the amplifier, thereby varying the time at which the scanning start pulse signal is generated.
Further, in the case where the light-sensitivity of a drum in use is not identical due to use of a different drum for respective cases, it is required to change the output or the intensity of the laser beam and adjust a threshold level for determining the time at which the scanning start signal is generated.